1. Field of the Invention
This invention relates to a thermal transfer line printer that is suitably used for forming an intermediate transfer type image in which ink of a multi-color ink sheet is transferred on an intermediate transfer sheet by means of a line thermal head to form a primary image and the primary image is retransferred on a transfer medium to form an image on the transfer medium.
2. Description of the Related Art
Heretofore, the intermediate transfer type thermal transfer line printer for forming an image on a transfer medium by means of a method in which ink of a multi-color ink film is transferred on an intermediate transfer medium by use of a line thermal head to form a primary image and the primary image is retransferred on a transfer medium by use of retransfer means has been used widely as an output apparatus of a computer or ward processor. The reasons are that a high quality printed image can be formed easily on various transfer media such as CD, CD-R, MO, DVD, and various cards in addition to regular paper and that the intermediate transfer type thermal transfer line printer is operated with low noise, available at low cost, and needs less maintenance.
A conventional thermal transfer line printer as described hereinabove operates as described hereunder. A line thermal head is brought into down-state in which the line thermal head is brought into contact with a platen roller with interposition of an ink sheet and an intermediate transfer sheet that are formed in the configuration of long sheet in this order in a primary image forming section. Heating elements of the line thermal head are heated selectively based on the printing information with feeding the ink sheet and the intermediate transfer sheet in this state to thereby melt or sublimate the ink carried on the ink sheet so that the ink is transferred on the intermediate transfer sheet, and a reverse image that is served as the intermediate image for one page, namely one picture, is formed on the intermediate transfer sheet. Thereafter, the intermediate transfer sheet is moved and the primary image formed on the intermediate transfer sheet is concomitantly moved to the position just before the retransfer section, and then the primary image is moved to the retransfer preparation position for start alignment with aid of the register mark formed on the intermediate transfer sheet so that the primary image is registered with a transfer medium. Then, the primary image formed on the intermediate transfer sheet is melted or sublimated by applying heat and pressure of retransfer means comprising a heating roller in a retransfer section, and the primary image is retransferred and fixed on the transfer medium to thereby form a desired image on the transfer medium.
At that time, in the case where a monochrome image of one color is formed on a transfer medium, one pass is enough for forming an image.
On the other hand, in the case where a multi-color image is formed on a transfer medium, an multi-color intermediate image is formed by means of so-called swing-back technique as described hereunder. A multi-color ink sheet on which a plurality of color ink regions are arranged so that different colors are repeated adjacently in the longitudinal direction is used as an ink sheet. At first, a reverse image of the first color ink that is carried on the multi-color ink sheet is formed on an intermediate transfer sheet. Then, a line thermal head is brought into head-up state in which the line thermal head is being separated from a platen, and the intermediate transfer sheet is moved reversely to the transfer preparation position in this state. The reverse image formed with the first color ink is returned to the transfer position for start alignment, and a reverse image of the next color is transferred on the reverse image of the first color one on the other.
In detail, in the case where a full-color image is formed, a multi-color ink sheet on which a plurality of ink regions, each of which consists of four colors, for example, K (black), Y (yellow), M (magenta), and C (cyan) are arranged so that the different colors are repeated adjacently in the longitudinal direction and which has color-discrimination marks on the boundary between different ink regions is used as an ink sheet.
More in detail, at first, a K-color reverse image for one page picture is formed on an intermediate transfer sheet by use of a K-color ink region on the multi-color ink sheet. Next, the intermediate transfer sheet that has been moved during the primary image forming operation is moved in the reverse direction to the transfer preparation position to align the K-color reverse image formed on the intermediate transfer sheet, and a Y-color ink region that is positioned adjacent to the K-color ink region of the multicolor ink sheet is aligned. With the use of the Y-color region in the multi-color sheet, a Y-color reverse image for one page picture is formed over the K-color reverse image for one page picture that has been formed on the intermediate transfer sheet. Similarly, reverse images of an M-color ink region and a C-color ink region are formed on the intermediate transfer sheet in this order to thereby form a full-color primary image for one page picture on the intermediate transfer sheet.
The register mark that has been formed on an intermediate transfer sheet previously or is formed simultaneously when a primary image is formed on an intermediate sheet. The primary image is formed on an intermediate transfer sheet so as to register with the register mark, and the primary image formed on the intermediate transfer sheet so as to register with the same register mark is retransferred so as to register with a transfer medium.
However, in the case of the above-mentioned conventional thermal transfer line printer, because a heating roller is in contact with pressure on a transfer medium, more in detail, on the head portion of the transfer medium with respect to the moving direction with interposition of an intermediate transfer sheet when retransferring is carried out, a load is applied abruptly on the intermediate transfer sheet. As the result, the portion that is in contact with the head portion of the intermediate transfer sheet with respect to the moving direction of the transfer medium on which a primary image has been formed is cockled, the tail portion with respect to the moving direction of the transfer medium is lifted upward from a tray surface due to the pressure force of the heating roller applied on the head portion with respect to the moving direction of the transfer medium, and a high quality image is formed on the transfer medium not consistently. If an intermediate transfer sheet is cockled, a primary image located on the cockled portion is not retransferred on a transfer medium to cause partial lack of image.
Furthermore, in the case of the conventional thermal transfer line printer, a register mark is detected in the state that a heating roller is being separated from an intermediate transfer sheet, an intermediate image on the intermediate transfer sheet is moved to the retransfer preparation position for start alignment, and then the heating roller is brought into contact with the intermediate transfer sheet. As the result, the tension of the intermediate transfer sheet changes at the portion that is in contact with the heating roller due to the load applied on the intermediate transfer sheet when the heating roller is brought into contact with the intermediate transfer sheet, the primary image that has been registered with the transfer medium deviates from the transfer medium, and a high quality image is formed on the transfer medium not consistently.
The present invention has been accomplished in view of the abovementioned problem, and it is the object of the present invention to provide a thermal transfer line printer that is capable of forming a high quality image on a transfer medium consistently.
To accomplish the abovementioned object, a thermal transfer line printer in accordance with the present invention is characterized by providing a controller for controlling a heating roller contact ON/OFF motor so that a heating roller contact ON/OFF mechanism is selectively operated at least in a weak pressure contact state that a heating roller is in contact with an intermediate transfer sheet with a weak pressure and in a strong pressure contact state that the heating roller is in contact with the intermediate transfer sheet with a strong pressure when retransfer is carried out. By employing the abovementioned structure, when retransfer is carried out, the pressure applied when the heating roller is in contact with the intermediate transfer sheet can be applied at least two-step wise so as to be weak at first and so as to be strong later. Thereby, the load change applied on the intermediate transfer sheet can be made gradual. As the result, cockling on the intermediate transfer sheet at the portion that is in contact with the head of the transfer medium in the moving direction is prevented, and lifting-up of the tail portion of the transfer medium in the moving direction from the tray surface is prevented. Therefore, a high quality image is formed on a transfer medium consistently.
It is preferable that the abovementioned thermal transfer line printer is provided with retransfer mark detection means for detecting a register mark formed on the intermediate transfer sheet when retransfer is carried out, and the controller controls an operation timing so that the heating roller is pressed with a weak pressure at a time before the retransfer mark detection means detects a register mark and the heating roller is pressed with a strong pressure at a time after the retransfer mark detection means detects the register mark when the retransfer is carried out. By employing the abovementioned structure, because a register mark can be detected in a state that the heating roller is in the weak pressure contact with the intermediate transfer sheet, when the retransfer is carried out, the positional deviation between the primary image and the transfer medium caused when the heating roller is brought into contact with the intermediate transfer sheet with pressure, which have been registered, due to a load applied on the intermediate transfer sheet is prevented consistently. Therefore, a high quality image can be formed on a transfer medium more consistently.